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/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
    let queue = []
    let res = []
    if(root===null) return res
    queue.push(root)
    while(queue.length!==0){
        let curLevel = []
        // 要这样拿,不能直接在for写queue.length,不然下面有push,会导致其越来越大
        let curLength = queue.length
        for(let i =0;i<curLength;i++){
            // 拿队头
            cur = queue.shift()
            curLevel.push(cur.val)
            if(cur.left!==null)
            queue.push(cur.left)
            if(cur.right!==null)
            queue.push(cur.right)
        }
        res.push([...curLevel])
    }
    return res
};